Many modern aircraft are equipped with an auxiliary power unit (“APU”) that generates and provides electrical and pneumatic power to various parts of the aircraft for tasks such as environmental cooling, lighting, powering electronic systems, and main engine starting. Typically, such APUs are located in the aft section of the aircraft such as the tail cone and are isolated by a firewall. During operation, an APU produces exhaust gas that is directed through a nozzle and out of the aircraft through an exhaust opening. The nozzle may communicate with an eductor system that utilizes the APU exhaust gas to draw and direct other gases through the aircraft.
To achieve this, eductor systems have been developed that include a first plenum (i.e. the oil cooler plenum) for drawing gas across an oil cooler, and a second plenum (i.e. the surge plenum) for directing surge flow to an exhaust duct (i.e. air not required by the aircraft to satisfy its pneumatic requirements, commonly referred to as surge bleed flow). During normal operation with no surge flow, the surge plenum is a dead-headed cavity with its aft facing outlet exposed to the mixed eductor flow; i.e. the turbine exhaust at perhaps 1000° F. and the cooling air from the cooling plenum at approximately 200° F. Thus, the mixed eductor flow, which may be about 500° F., enters the surge plenum, circulating in and out of the surge plenum, and heating the surge plenum to approximately 500° F., exceeding the strict temperature limits (i.e. 450° F.) being imposed on the outer surfaces of the APU including the surge plenum.
In accordance with the forgoing, it would be desirable to provide a system and method for directing a cooling flow into the surge plenum to reduce the temperature of the surge plenum surfaces when there is no surge flow.